Understanding the genomic distribution of mammalian recombination is a matter of considerable importance for 1) optimizing strategies for association studies mapping human disease genes by linkage disequilibrium, 2) optimizing experimental strategies for mapping and cloning genes in experimental mammals, 3) understandingthe role of recombinational hotspots in evolutionary processes, and 4) under- standing genetic recombination as a biological process. The purpose of the proposed research is to provide a molecular description of the locations and properties of hotspots in a sample of the mouse genome. We will map to a resolution of 1.5 Kb, the locations of 900-1000 crossovers occurring in five 5-megabase regions of the mouse genome, selected for specific characteristics. The crossovers, which we expect to describe nearly 80 recombinational hotspots, will be identified in DNAs from (C57BL/6J x CAST/EiJ) F2 progeny. Our hotspot characterizations will provide the locations and recombination frequencies of all hotspots down to a level of 0.02 cM/hotspot, and will include the haplotype and sex specificity of each hotspot. To our knowledge, this will be the first such large-scale effort for a mammal. Informatics studies will attempt to identify key hotspot sequences and chromosomal features that define hotspot locations. Additionally, for one of the chromosomal regions, we will carry out a genetic test for the existence of trans-acting genes required for hotspotactivity.